One of the many challenges facing the preterm infant is synchronizing activities with the normal, 24-hour day. There is convincing evidence linking the day-night patterns of body temperature and sleep-wake activity in adults however, little is known of this relationship in preterm infants. Preterm infants, because of their thermoregulatory limitations, require heating by incubators or other clinical devices. There has been little investigation of how such heating may alter sleep-wake pattern. This relationship is significant to the preterm infant's adaptation to the 24- hour day-night pattern. Infant rhythmicity and the ability to synchronize with caregiving patterns are factors salient to infant-parent interaction. Nursing management of incubators, based on recognition of the preterm infant's developing body temperature biorhythm, is a potential intervention for promoting sleep-wake pattern. The purpose of this research is to examine the relationship between temperature and sleep-wake state in preterm and term gestation infants. Specific aims of the study are to examine the development of temperature biorhythm in hospitalized infants born prematurely, determine the relationship between preterm infant sleep- wake state and both infant body and incubator temperature, and to compare body temperature biorhythm and sleep-wake state and both infant body and incubator temperature, and to compare body temperature biorhythm and sleep- wake patterns in preterm and term infants in the home environment. Preterm infants will be studied longitudinally and cross-sectionally using an intensive within subject design. Incubator temperature and infant abdominal skin temperature will be recorded during a 24-hour period using a computerized data acquisition system while infant state will be determined by lapse-time video recording. Following discharge, preterm infants and a comparison group of term infants will studied in the home environment where infant abdominal skin temperature (Vitalog) and infant sleep-wake state will be measured. Cyclicity in both body temperature and sleep-wake states will be determined using power spectral analysis. Comparisons of preterm infants across time and cross-sectionally will reveal developmental changes in biorhythm of body temperature and sleep-awake states. The effect of premature birth on the development of temperature and sleep-wake state patterns will be evidenced in comparisons of term and preterm infants.